1. Field of the Invention
The present invention relates to a sensor for detecting methane gas and propane gas, which has tin oxide (SnO.sub.2) thin film instead of thick film or bulk type of tin oxide conventionally used for detecting reductive inflammable gas [e.g. propane (C.sub.3 H.sub.8), butane (C.sub.4 H.sub.10)], and which can detect methane or propane gas at very low temperature.
2. Description of the Conventional Art
Generally, methane gas has lower specific gravity (0.554) than propane(1.52) or butane gas(2.00). As methane gas is lighter than the air, it rises to the upper side of the room when it is leaked. Also, it is less explosive than other gases as the explosive minimum concentration of methane gas (5.00%) is higher than other gases (propane: 2.12%/butane: 1.80%) [KIM, Young Hae, Gas Sensors and Application thereof, Kijeon-Younku-Sa, 1993, p.6].
In these circumstances, researches for applying methane gas as fuel are increasingly performed recently in order to develop a stable and clean fuel. Thus, a development of a sensor for detecting the leakage of methane gas as mentioned above and of propane gas which is widely used as fuel is urgently requested.
As conventional sensors for gas detection have had a type of bulk or thick film rather than thin film, gas sensitivity was lower than that of a thin film type sensor, and was disadvantageous in that production in large scale was difficult owing to hand-working, to cause high price of the product.
As a sensor for inflammable gas presently used in Korea, a bulk type sensor based on SnO.sub.2 imported from Japan may be mentioned. However, it is disadvantageous in that it requires much labor for its mass production and has low selectivity, while a process of heat treatment at high temperature must be involved in its production.
Meanwhile, as the conventional methane sensors and propane sensors have very high operation temperature (more than 400.degree. C. and more than 300.degree. C., respectively) to cause high electric power consumption, and a selective sensor has not yet been disclosed because methane gas has a stable chemical structure [Kiyoshi Fukui and Masanori Nakane, Effects of oxide semiconductor-electrode interface on gas-sensitivity characteristics, Sensors and Actuators B, 15-16 (1993), 24-31].
Because of the difficulties of selecting a sensor for detecting methane gas, a method for indirectly estimating the presence of methane gas by combining two or more sensors has been reported [Pascale Durone et al., A new approach to selectivity in methane sensing, Sensors and Actuators B, 15-16(1993), 24-31]. However, this method is an indirect estimating method and requires a complicated electronic device owing to the use of two or more sensors.
On the other hand, a sensor of thin film type having a thickness of several hundreds to several thousands .ANG. may result in improved sensitivity, reproductivity and mass productivity as well as minimizing electric power consumption by virtue of miniaturization, whereby lowering the price. Up to the present, however, any research or invention with regard to a thin film type sensor which can provide sufficient selectivity to be applied to detection of methane or propane gas has not been reported.
Recent studies are focused to an addition of rare element such as platinum or palladium to a prepared thin film in order to increase the sensitivity of SnO.sub.2 sensor. Though developments of a novel process for preparing thin film to which rare element can be easily added are intended, it has not yet been developed inside or outside the country. This is because most of the researches tried to prepare the thin film by physical sputtering or chemical vapor deposition, by which controlling of the thickness, orientation, crystallinity, density or ultrafine processing is unable or very difficult in practice.
In the preparation of the thin film type sensor, a method using ion beam is known to be able to afford a thin film having high quality and high density during the low temperature process through all kinds of metals, semiconductors and oxides [S. K. Koh et al., Effects of Residual Gas on Cu Film Deposition by Partially Ionized Beam, JVST-A, 13(4), 1995, 2123-2127].
According to the present invention, SnO.sub.2 thin film can be formed at room temperature by vaporizing neutral Sn metal by the use of metal gun, with ionizing oxygen (O.sub.2.sup.30) by the use of a gas ionized gun, and applying energy thereto.
The principles and characteristics of the gas ion source mentioned above is suggested in a disclosed article of the present inventors [S. K. Koh et al., Characteristics of a Metal Ion Source for Fabricating Sn Metal in an O.sub.2 Environment, Ungyong Mulli, 9(2), 1996, 246-252.